Thermoelectric materials: recycling energy

For some years now, NASA has been using what are called
thermoelectric materials to power its space probes. The probes travel such
great distances from our sun that solar panels are no longer an efficient
source of power. So NASA imbeds a nuclear material in a radioisotope thermal
generator, where it decays, producing heat energy. That energy is then
converted by thermoelectric materials into the electricity that powers the
space probe. The same technology is now being explored for more earthly
applications, for example, to capture heat lost in the exhaust of automobiles
to produce electricity for the vehicle.

Thermoelectric materials are a hot new technology that is
now being studied intensively by researchers funded by the U.S. Department of
Energy's Energy Frontier Research Centers. Oliver Delaire, a Shull fellow at
ORNL, is part of such a collaboration, this one led by an EFRC at the
Massachusetts Institute of Technology. Delaire uses neutron scattering and
computer simulation to investigate the microscopic structure and dynamics of
thermoelectric materials so that researchers can make them more efficient for new,
energy-saving applications.

Thermoelectrics are adaptable for both heating and for
cooling applications. These materials can convert low-grade heat that is wasted
in an industrial process, or in the exhaust system of a vehicle, into
electricity. Or they can transport heat from an external source of power and manipulate
it to cool a surface.

But there are limitations in the materials themselves.
"Right now the materials may be on the order of 10% efficiency,"
Delaire said. "If we can make them two or three times better, if we can
get 30% efficiency, that would get people very excited, and it would be much
more viable economically."

The researchers want to improve their understanding of
phonons, the atomic vibrations that transport heat through thermoelectric
materials. "Neutron scattering is unmatched in its ability to probe the
atomic vibrations in the crystals," Delaire said. "This is one of the
fundamental blocks in the process that we need to understand better."

At ORNL, Delaire and his collaborators are using the Time of
Flight spectrometers at SNS-ARCS and CNCS-and the HB-3 Triple-Axis Spectrometer
at the High Flux Isotope Reactor.

"The SNS instruments offer more power," Delaire
said. "They can sample the whole parameter space very efficiently. They
offer a unique opportunity in a single experiment to sample all the types of
atomic vibrations, all the phonons inside the solid. And once we have this
information, we can reconstruct the microscopic thermoconductivity, which is
really the property we are trying to understand."

The single crystals are grown at ORNL. There are two
high-temperature materials, PbTe (lead telluride) and La3Te4 (lanthanum
telluride) for heat recovery and FeSi (iron silicide) for refrigeration
applications. Single crystals offer advantages in neutron science. They can be
measured on the TOF instruments at a series of orientations, each orientation
giving a comprehensive data set. Combining these sets gives the researchers a
complete picture, much like tomography.

"By doing this experiment using the high neutron flux
here at SNS we are able to really map out the space, and that is really where
the big advance is," Delaire said. "Previously we were only able to
look at a few points in the space, and that is like looking with a flashlight.
With the flux we have at SNS, we turn on the overhead lights and see the whole
room all at once."

Once they have a view of the volume of data, the researchers
then use the triple-axis spectrometer at HFIR to zero in and look at pinpoints
of phonon vibrations in that region. Next, ab initio computational methods
simulate the data that the instruments have generated. These simulations are
based on quantum mechanics, which gives the researchers a direct comparison
between the neutron science measurements and fundamental theory and theoretical
predictions.

In
2010, the researchers uncovered an important coupling between thermal disorder
and the electronic structure of FeSi. This type of coupling between thermal
disorder and electronic structure at finite temperature could prove to be
important in many materials, including thermoelectrics. The research is funded
by an Energy Frontier Research
Center at MIT and
involves a dozen investigators, including Delaire and David Singh, a solid-state
theorist in materials science at ORNL. Samples are also contributed by
researchers at Boston
College. Delaire, Singh,
and some of the MIT researchers are involved in the computation.